Reliability work occupies an increasingly important place in engineering practice. Although the details differ depending on whether mechanical, electrical, chemical, or other systems are under analysis, the reliability concepts and the mathematical foundations cut across the specific fields of application. Over the past 50 years thousands of papers and dozens of books on mathematical models of reliability have been published. A comprehensive survey alone on the current developments in the mathematical theory of reliability would fill a voluminous book. Based on importance both for theory and application and taking into account the interests of the author, current investigations in four important branches of reliability theory are considered: coherent systems, stochastic networks, software reliability, and maintenance theory. *) Invited plenary lecture at the Annual Conference of the German Statistical Society and the Stochastic Society within the German Mathematical Association, Wittenberg (Germany), May 1997. http://orion.journals.ac.za/